Although so-called radiography employing silver halide photography is conventionally used to obtain radiographic images, there also has been developed a radiographic imaging method not using silver halide photographic material. Thus, an imaging method is disclosed, in which radiation that has been transmitted through an object is absorbed by phosphor, followed by exciting the phosphor with energy to cause a radiation energy accumulated in the phosphor to radiate in the form of fluorescence, and imaging is achieved by detecting the fluorescence.
Specifically, U.S. Pat. No. 3,859,527 discloses a radiation image conversion method, in which a panel having on a support a stimulable phosphor layer is employed using either or both visible light and infrared rays as the stimulating energy.
There have been developed radiation image conversion methods using a stimulable phosphor exhibiting enhanced luminance and high sensitivity, including, for example, a radiation image conversion method employing BaFX:Eu2+ type phosphor (X: Cl, Br, I), as described in JP-A No. 59-75200 (hereinafter, the term, JP-A refers to Japanese Patent Application Publication); a radiation image conversion method employing an alkali halide phosphor, as described in JP-A No. 61-72087; a radiation image conversion method employing an alkali halide phosphor containing, as co-activators, Tl+ and metals such as Ce3+, Sm3+, Eu3+, Y3+, Ag+, Mg2+, Pb2+ or In3+, as described in JP-A Nos. 61-73786 and 61-73787.
Recently, a radiation image conversion panel exhibiting further enhanced sharpness has been desired in the field of diagnostic image analysis. Of these, an attempt in controlling the form of stimulable phosphor grains to enhance sensitivity and sharpness was made as a means for improving sharpness of radiographic images.
For example, JP-A No. 61-142497 discloses a method of using a stimulable phosphor layer comprising a fine pseudo-columnar block which has been formed by sedimentation of a stimulable phosphor on a support having fine protruded patterns.
JP-A No. 61-142500 discloses a method of using a radiation image conversion panel having a developed stimulable phosphor layer by applying a shock treatment of cracks which appeared in columnar blocks formed by sedimentation of a stimulable phosphor on a support having fine patterns; JP-A No. 62-39737 discloses a method of using a radiation image conversion panel having a stimulable phosphor layer having a pseudo-columnar form which has been formed by producing cracks on the layer surface side; JP-A No. 62-110200 proposes a method in which a stimulable phosphor layer having voids is-formed by vapor deposition onto the upper surface of a support, followed by growing voids by subjecting a heating treatment to produce cracks.
JP-A No. 2-58000 proposed a radiation image conversion panel having a stimulable phosphor layer, in which long and thin columnar crystals were formed with an incline at a given angle toward the direction normal to the support.
In the foregoing attempts to control the form of a stimulable phosphor layer, it was intended to enhance image quality by allowing the phosphor layer to have a columnar crystal structure. It was supposed that the columnar form prevented traverse diffusion of stimulated emission light (or photo-stimulated luminescence), i.e., the light reached the support surface with repeating reflection at the interface of cracks (or columnar crystals), thereby leading to markedly enhanced sharpness of images formed by the stimulated luminescence.
Recently, a radiation image conversion panel using a stimulable phosphor containing an alkali halide such as CsBr as a basic substance and Eu as an activator, and the use of activator Eu leading to enhanced X-ray conversion efficiency, which has formerly not been achieved.
A desired, high X-ray conversion efficiency, however, can not be obtained and it is not put into practical use on the market, because there is a problem that Eu in a basic substance can not be diffused homogeneously because of the high vapor pressure under vacuum and the remarkable thermal diffusion of Eu.
But, the film formation with an evaporation technique is desired to prepare CsBr:Eu phosphor layer and a film thickness distribution can be achieved in a high degree of accuracy by accurately positioning a substrate and an evaporation source and properly arranging a physical position in order to control the film thickness distribution.
The evaporation method and homogeneity in the film influenced by the base material have become important, because there is a problem that Eu in a basic substrate can not be diffused homogeneously because of the high vapor pressure under vacuum and the remarkable thermal diffusion of Eu especially in CsBr:Eu.
A uniform adhesion between the base material and a phosphor layer is necessary for better performace obtained by having a thick film because of a large scale in area. Change in a degree of vacuum caused by generation of volatile ingredients at the time when a phosphor film is deposited results in inferiority in uniform thickness and inhomogeneity of the activator contained in the phosphor layer, because there are a lot of residual solvents and volatile ingredients, depending on a manufacturing process of a resin, in the case of the resin prepared on a surface of the base material.